It's obviously true that poverty is about more than income, so why did I feel uneasy about Frank Field putting this undeniable claim at the heart of his report last week? Poor health, poor schooling and – above all – the question of parenting, on which Field lays such emphasis, are indeed important. My problem is that money also matters, as Joseph Rowntree Foundation analysis has underlined only this week. And the poor get poorer the moment interest in their financial position fades. That was true in the 1980s when child income poverty doubled, and it is true in spades now that the coalition has severed the link between social security and the real cost of living. By including groceries while ignoring costly housing in the calculation of benefit rates, it has imposed a literal bread-line that is abjectly inadequate as a guide to the price of getting by in modern Britain.

David Cameron, who bought into Blair-Brown rhetoric on child poverty to show that his Conservatives had changed, may now use this report to wriggle out of the specific target that accompanied the warm words: reducing the number of children stuck in households with less than 60% of median income. Isolation, squalor and depression are all forms of hardship, but few who have suffered a sustained lack of income would dispute that it makes all these things worse. Neither, for that matter, would Field. He merely wants to see any available extra money split between cash benefits and "foundation years" services for young children, such as children's centres and nurseries, institutions that he says should be contractually required to narrow the economic gap.

It sounds great, and the only possible objection to any of it is context – but there are times when context is everything. The coalition has removed funding guarantees for many such services, and refused to implement inherited legislation that would have required public authorities to promote class equality. Above all, it has fixed on expenditure plans that allow no room for choices about extra investments, only a choice between cuts.

Now, in principle, one would want to evaluate alternative cuts packages in the same even-handed manner as would be appropriate in choosing between alternative expenditure increases. It is possible, too, that Field is right to believe that social problems originally caused by financial hardship may have grown too complex to be solved by financial means – just as heart disease brought on by lack of exercise may not be cured by a jog. Even so, at times like these the incomes of the disenfranchised are especially vulnerable, and require the protection of a hard policy target.

Alternatives to low income, such as the social mobility talked up by the deputy prime minister, Nick Clegg, lack political bite because generational movements occur too slowly to bear on a five-yearly electoral cycle. To be fair to Field, he has proposed nine or 10 specific indicators that could gauge the mechanisms that hold poor children back earlier. The simultaneous use of so many metrics, however, could allow neglectful ministers to hide in a statistical fog.

Even though the last government missed the old poverty target by a country mile, the attempt to hit it did much good. International experts, such as America's Jane Waldfogel, rank the lifting of 600,000 children out of relative poverty over the last dozen years as a startling achievement by global standards. Britain's war on cash poverty may also have started to stir social mobility: the cast-iron link between parental finances and school performance has rusted a little.

A wider target to reflect the full width of the poverty problem would be positively desirable if we trusted the government not to skimp on the cash. But campaigners demanded a hard income target because they never extended such trust to Tony Blair. After £18bn in benefit cuts, they will be incomparably more reluctant to trust the coalition.